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Hess’ Law: Adding Reaction Enthalpies

Hess’ Law: Adding Reaction Enthalpies. Some chemical reactions can not be carried out easily in order to determine the amount of heat or energy that can be obtained. Eg : Mg (s) + 1/2O 2(g)  MgO (s) + light ?.

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Hess’ Law: Adding Reaction Enthalpies

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  1. Hess’ Law: Adding Reaction Enthalpies Some chemical reactions can not be carried out easily in order to determine the amount of heat or energy that can be obtained. Eg: Mg(s) + 1/2O2(g)  MgO(s) + light ?

  2. G.H. Hess recognized a relationship between algebra and chemical equations. Chemical reactions can be added, subtracted, multiplied, and divided like math equations. For any chemical reaction that can be written in a series of steps, the overall enthalpy of the reaction is the result of adding up the sum of all the individual steps of the reaction.

  3. Eg: Calculate the enthalpy of reaction for the formation of rust. 2Fe(s) + 3/2O2(g)  Fe2O3(s) Given: • Fe2O3(s) + 3CO(g)  2Fe(s) + 3CO2(g) DHo= -26.7 kJ • CO(g) + 1/2O2(g)  CO2(g)DHo= -282.7 kJ We can rearrange the given equations in a manner that will yield the overall reaction.

  4. X-1 2Fe(s) + 3CO2(g)  Fe2O3(s) + 3CO(g) DHo= +26.7kJ • X3 3CO(g) + 3/2O2(g)  3CO2(g) DHo=-848.1kJ _____________________________________________ 2Fe(s) + 3/2O2(g)  Fe2O3(s) DHo=-821.4kJ

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